1. Field of the Invention
The present invention relates to a photonic band gap fiber wherein multiple air holes are provided in silica portions along the longitudinal direction of the fiber. The photonic band gap fiber of the present invention can inhibit surface modes specific to ordinary photonic band gap fibers and can expand the transmission bandwidth of the fiber. It can, therefore, be used in very low loss optical transmissions, optical transmissions from the UV region to the visible light region and infrared region, and in fiber laser optical transmissions.
2. Description of Related Art
By using a periodic structure of air holes in the cladding, a photonic band gap fiber (hereinafter referred to as “PBGF”) can confine light in the core by making use of its photonic band gap. Even if the core is air, wave guidance is possible. (See R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. St. J. Russell, P. J. Roberts, and D. C. Allan, “Single-mode photonic band gap guidance of light in air,” Science, vol. 285, no. 3, pp. 1537-1539, 1999.)
However, even if the periodic structure of air holes provided in the cladding forms a band gap, the core mode wherein light is concentrated in the core couples with the surface mode wherein light is concentrated in the silica close to the core edge and causes a large transmission loss. Thus, optical wave guiding cannot be obtained for the entire wavelength band of the band gap (see J. A. West, C. M. Smith, N. F. Borrelli, D. C. Allan, and K. W. Koch, “Surface modes in air-core photonic band-gap fibers,” Opt. Express, vol. 12, no. 8, pp. 1485-1496, 2004).
The presence of a surface mode depends on the magnitude of the core diameter. FIG. 1 shows this dependency.
The conventional PBGF 1 shown in FIG. 1 comprises multiple circular air holes 11 in a triangular lattice configuration in the silica portion 10 seen in the cross section of the fiber. The air hole at the center forms an air hole core 12. The structure of air holes forming a triangular lattice periodic structure with multiple circular air holes 10 arranged at constant pitch in the cross section of fiber in this way is referred to hereinafter as a “normal triangular lattice periodic structure.”
In FIG. 1, “bulk mode” refers to a mode with Γ points (points at which the wavelength vector has components only in the transmission direction), which has maximum frequencies in the low pass band of the band gap, when the periodic structure of air holes constitutes the band gap.
In the PBGF of the structure as shown in FIG. 1, it is well known that a surface mode is present when the edge of core 12 cuts through the bulk mode, and a surface mode is absent when it does not cut through the bulk mode (see H. K. Kim, J. Shin, S. Fan, M. J. F. Digonnet, and G. S. Kino, “Designing air-core photonic-bandgap fibers free of surface modes,” IEEE J. Quant. Electron., vol. 40, no. 5, pp. 551-556, 2004).
FIGS. 2 and 3 are examples of the positional relationship between an air hole core 12 and a bulk mode in the conventional PBGF 1 with a normal triangular lattice periodic structure. The conventional PBGF 1 shown in FIG. 2 comprises multiple circular air holes 11 arranged in a triangular lattice configuration in the silica portion 10 that forms the cladding in the cross section of the fiber. An air hole core 12 is provided formed by air holes in a region that includes six air holes surrounding one central air hole. The conventional PBGF 1 shown in FIG. 3 comprises multiple circular air holes 11 arranged in triangular lattice configuration in the silica portion 10 in the cross section of the fiber. An air hole core 12 is formed by one air hole at the center surrounded by 18 air holes in two layers.
However, when a normal triangular lattice periodic structure as shown in FIGS. 2 and 3 is used in the cladding, the edge of the air hole core 12 cuts through the region wherein bulk mode 13 is present; therefore, it becomes difficult to avoid the surface mode. The result is that the light in the core mode couples with the surface mode, causing large transmission loss, such that optical wave guiding throughout the wavelength band cannot be obtained, the wave guide bandwidth becomes narrow, and the transmission loss increases further.